No, I didn't
For a while I haven't had the "Achievements" update, and I don't like how it looks - so I was fine with that. Duolingo wasn't, apparently. I have them now.
The first Achievement it said I got was "Champion" for completing a course. I have not completed any courses whatsoever. I'm close on Japanese but that's it.
Why did I get this achievement?
you mean you haven't completed a single course? like it counts also any previous courses you've done
I know that. I haven't completed any course whatsoever on any account in any language.